6-Alkyl- (and aryl-) -3-mercapto-4-amino-1,2,4-triazin-5-ones are known and, with their various derivatives, are an important class of herbicides (U.S. Pat. No. 3,966,715; Chem. Ber. 97, 2173 (1964). The 6-carboxy structures are described in my copending application Ser. No. 6/238,480 filed 2/26/81. It is further noted that tert.-butyl derivatives are of particular activity by virtue of their inhibitory effect on enzyme reactions. An important example is metribuzin (Merck Index No. 6027).